MLXKIIALOGY 



CHAPTER I 

 CRYSTALLOGRAPHY 



THE solid, liquid, and gaseous states of matter depend upon 

 temperature and pressure. It is possible to cause a solid to pass 

 to the liquid state by raising the temperature. At C. water 

 passes from the solid ice to the liquid water; C. is the fusing 

 point, or the temperature at which the solid passes over to the 

 liquid water. If the temperature is increased until 100 C. is 

 readied, water passes to the gaseous state steam. The tem- 

 perature at which the vapor passes off freely and where there is no 

 further rise in the temperature of the body of the liquid is known 

 as the boiling point. The fusing point and the boiling point are 

 fixed temperatures for pure chemical compounds. Upon decreas- 

 ing the temperature and increasing the pressure sufficiently all 

 substances become solid. If the solids formed by a slow transi- 

 tion from liquids or gases are examined, it will be found that the 

 larger number are bounded, in part at least, by smooth plane 

 faces. When arsenious oxide is heated it volatilizes; the vapors 

 upon contact with a cold surface condense, forming a white coat- 

 ing ; on examination with a lens, this white coat is found to be 

 composed of small particles bounded by eight triangular faces, 

 Fig. 1. Each individual is a crystal of arsenious oxide. All 

 polyhedra formed by substances when passing to the solid state 

 are crystals. 



It has been the conception of scientists, since the time of Dalton, 

 that the fundamental unit of matter was the atom ; that the num- 

 ber of kinds of atoms is limited, and that each kind possesses dis- 

 tinct properties separating it from all others, thus forming a 

 simple substance, an element. While the number of different 

 kinds of atoms is small, all objects and compounds of nature are 

 possible by the combination of this small number of elements. 



The number of atoms joining to form the unit or molecule of a 

 compound can, by the law of Avogadro, be determined for all 



